It's vulnerable to a known-plaintext attack. Eg, you have an account on the system, so you know your own password. You can encrypt your password with differen

It's vulnerable to a known-plaintext attack. Eg, you have an account on the system, so you know your own password. You can encrypt your password with different keys until it matches the stored, encrypted version. Then you know the master key. Edit: Of course you did say 128 bits, which is pretty good. But if you do manage to break it, you do have the master key, which can't happen with hashed passwords.